Recycling a homogeneous catalyst through a light-controlled phase tag

Article abstract of DOI:10.1002/anie.200906034

(Figure Presented) A homogeneous ruthenium-carbene complex was tagged with a lightresponding nitrobenzospiropyran group to control its solubility through reversible conversion between its neutral (lipophilic) and ionic (lipophobic) states using light irradiation (see scheme; Mes = mesityl). This tagged complex has significant catalytic activity in ring-closing metathesis reactions and was recycled several times.

Full text of DOI:10.1002/anie.200906034

Angewandte
Chemie
DOI: 10.1002/anie.200906034
Homogeneous Catalysis
Recycling a Homogeneous Catalyst through a Light-Controlled
Phase Tag**
Guiyan Liu and Jianhui Wang*
Homogeneous catalysts, which exist in the same phase as the
reactants, are known to have superior behavior and selectivity
compared to heterogeneous catalysts. However, recovery of a
catalyst from a reaction has remained a fundamental problem
as separating it from the end products is usually costly and
inefficient.[1] To overcome tedious separation processes,
several catalysts such as interphase,[2] clathrate-enabled,[3]
ionic-tagged,[4] fluorous-tagged,[5] redox-switchable phase
tagged,[6] and solid- or polymer-supported catalysts[7] have
been developed. Some of these methods,[8–12] such as aqueous
biphasic catalysts for propene hydroformylation, phase-
switchable catalysts using CO2 for the switch, as well as
other catalysts have been so successful that some of them
have been commercialized.[8,9] Strategies involving ionic-
tagged catalysts that can be used for olefin metathesis are
also attractive because these catalysts can be easily repeatedly
recycled from the reaction mixture.[13] However, these
systems require expensive ionic liquids as the support
materials, result in heterogeneity, and thus slows down the
reaction rate in the subsequent reactions. In addition, these
methods often result in impurities in the recycled catalysts,
such as decomposed catalyst, which may accelerate the
catalyst decomposition in future reactions.
The catalytic activity of Ru catalysts influenced by light
has been reported recently.[14] Herein, however, the use of a
light-controlled phase tag to separate homogeneous catalysts
from their reaction products is demonstrated. These types of
light-controlled phase tags can be switched between a neutral
(lipophilic) phase and a charged (lipophobic) phase through
the use of a tag-centered photoreaction. The photoreaction
results in drastic changes in the polarity and solubility of the
catalyst. Thus, it is possible to optimize the solubility of the
catalyst in different solvents and thereby improve extraction
and separation of the catalyst from the products.
chosen as the light-active group because it undergoes rapid
and reversible photo transitions (with high quantum yield)
between a colorless spiro (SP) state and a colored merocya-
nine (ME) state without generating photoproducts.[16,17]
Scheme 1 illustrates the synthetic route for the (R/S)-SP-
tagged Ru complex 6. The (R/S)-SP tagged ligand 5 was
synthesized in a reaction that produced moderate to good
yields using 4-bromo-1-isopropoxy-2-propenyl-benzene (2) as
the starting material. Treatment of 2 with the second-
generation Grubbs catalyst in the presence of CuCl with
CH2Cl2 as the solvent at 408C, as described by Hoveyda and
co-workers,[15a] resulted in the exchange of the styrene group
and produced a good yield (64.7%) of the (R/S)-SP-tagged
Ru complex 6 as a green crystalline solid.
As expected, irradiation of complex 6 with light caused
the (R/S)-SP tag to convert from the neutral (lipophilic) phase
to the charged (lipophobic) phase, thus forming complex 7. In
the absence of light complex 7 converted back into complex 6.
A typical experiment to demonstrate this conversion was
conducted in CH3CN. When irradiated with light (l >
380 nm), an absorption band at 563 nm appeared in the UV
spectra, thus indicating the formation of trans-ME (Fig-
ure 1a). Then after two minutes the absorption peak reached
a maximum, thus indicating the complete conversion of (R/S)-
SP into trans-ME. In the absence of light complex 7
completely converted back into complex 6 as indicated by
the disappearance of the absorption band at 563 nm (Fig-
ure 1b).
To demonstrate this new concept, the Hoveyda–Grubbs
boomerang-shaped ruthenium–carbene complex 1[15a] was
selected as the catalyst precursor because it is a known
effective catalyst.[15] Furthermore, numerous recycling meth-
ods have already been tested using this high-value cata-
lyst.[4a,b,5b,6a,b,d] A nitrobenzospiropyran ((R/S)-SP) unit was
Further study indicated that the aforementioned tag-
centered reaction was significantly affected by the solvent
(Figure 1c and d). The reaction was very rapid, reaching its
peak within 2 minutes in light when cyclohexane and CH3CN
were used as the solvents. Whereas the reaction rate of
complex 7 completely converting back into complex 6 was
fastest when CH2Cl2 was the solvent. The transformation was
completely reversible in cyclohexane, CH3CN, and CH2Cl2
and was repeated eight times.
[*] G. Liu, Prof. J. Wang
Chemistry Department, College of Science
Tianjin University, Tianjin, 300072 (China)
Fax: (+86)22-27892497
E-mail: wjh@tju.edu.cn
[**] This work was supported by the NSFC (20872108) and Tianjin
University.
Significantly, complexes 6 and 7 have remarkable dissim-
ilar characteristics of solubility as a result of the different
polarities of the (R/S)-SP and trans-ME tags. Hence, the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906034.
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subsequently completely dissolved in
the polar phase (Figure 2b: lower
layer). In the absence of light and at
room temperature, the ionic trans-ME
tag converted back into the neutral (R/
S)-SP tag and produced complex 6.
Afterwards, 6 shifted back to the cyclo-
hexane layer. The cycle was repeated
seven times, thus proving that the usage
of light to transform complex 6 into 7
between the two solvents is significantly
reversible (Figure 2c).
The (R/S)-SP-tagged ruthenium–
carbene 6 is a highly active catalyst for
ring-closing metathesis (RCM) reac-
tions. Kinetic studies using diethyl dia-
llymalonate 8 as a test diene indicated
that under similar reaction conditions,
Scheme 1. Synthesis of (R/S)-SP-tagged ruthenium–carbene complex 6. DCC=1,3-dicyclohexyl-
carbodiimide, DMAP=4-dimethylaminopyridine, Mes=mesityl.
Figure 2. Shuttling of complex 6 between a two-phase system contain-
ing cyclohexane, glycol, and methanol (3:2:1, by volume) through a
light-controlled tag-centered reaction: a) In the absence of irradiation.
b) After irradiation with light. c) Concentration of complex 6 in the
upper layer; 1) upper solution (by UV analysis); 2) lower solution (by
ICP-MS analysis); 3) upper solution (by ICP-MS analysis); 4) lower
solution (by UV analysis).
the catalytic activity of 6 is comparable to that of complex 1.
A high conversion (> 98%) was achieved after 1 hour for the
RCM of 8 at 308C (see Figure S1 in the Supporting
Information). Further study showed that the ring closure of
N-protected substrates 10, 12, 14, 16, and 18, sulfur-containing
substrate 20, or oxygen-containing dienes 22, 24, and 26 led to
either five-, six-, or seven-membered rings with a di-, tri-, or
tetra-substituted double bond, respectively (Table 1,
entries 1–9). The products were obtained in high yield by
using a small loading of complex 6 (0.5–2.5 mol%). Cross
metathesis of olefins 28 and 29 also produced high yields
when 2.5 mol% of complex 6 was used (Table 1, entry 11).
Good yields were also obtained in ring-closing reactions of
olefins 31 and 33 through enyne metathesis (Table 1,
entries 12 and 13). Thus, complex 6 is a highly active catalyst
for a wide range of RCM reactions. Moreover, it is active with
many functional groups which is similar to its parent
compound 1.
Figure 1. Evolution of the absorption spectrum for the catalyst (1.0ꢀ10À4 m,
258C: a) Irradiation with light (l>380 nm) in CH3CN. b) In the absence of
light in CH3CN. c) Irradiation of the sample in CH3CN, CH2Cl2, and cyclohex-
ane. d) Sample in different solvents in the absence of light.
solubility of the two complexes can be adjusted by switching
the (R/S)-SP tag by irradiation. A conventional experiment
that exhibits the aforementioned concept was conducted in a
biphasic system containing cyclohexane as the nonpolar
solvent and a mixture of glycol and methanol (3:2:1, by
volume) as the polar solvent (Figure 2). Complex 6 with a
neutral (R/S)-SP tag showed very good solubility in cyclo-
hexane (Figure 2a: upper layer). Only traces of the former
compound, complex 6“, were recovered from the polar
solvent mixture (lower layer). When the sample was irradi-
ated with light, the (R/S)-SP tag immediately transformed
into the ionic trans-ME tag to produce complex 7, which,
In contrast to other tagged or supported catalysts, the
(R/S)-SP-tagged complex 6 can be easily recycled by manip-
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Table 1: Metathesis and recycle study of catalyst 6 for different dienes.[a]
Entry Diene
Product
6 [ꢀ10À4 mmol]
(Ru [mol%])
Convn. [%][b]
(yield [%])[c]
Remaining 6[d]
Recovered 6[e]
[ꢀ10À4 mmol]
(rate [%])[f]
[ꢀ10À4 mmol]
1
2
3
4
5
0.5 (4.8)
0.5 (4.8)
0.5 (4.8)
0.5 (4.8)
2.5 (24.2)
>98 (95)
>98 (97)
>98 (97)
>98 (97)
>95 (93)[g]
4.0
4.2
3.9 (96)
4.1 (97)
4.1 (97)
4.2 (94)
16.3 (96)
4.2
4.5
16.9
6
7
1.0 (9.7)
0.5 (4.8)
>98 (97)
>98 (96)
8.4
4.3
7.8 (94)
4.1 (96)
8
9
0.5 (4.8)
0.5 (4.8)
>98 (94)
>95 (90)
4.4
4.3
4.2 (95)
4.1 (96)
10
1.0 (9.7)
>90 (87)
7.2
6.9 (95)
11
12
13
2.5 (24.2)
2.5 (24.2)
1.0 (9.7)
>90 (85)
>90 (85)[h]
>85 (80)
17.9
18.1
6.1
16.8 (94)
17.2 (95)
5.9 (97)
[a] Reactions were conducted at 358C in CH2Cl2. [b] Conversion of substrate was determined by 1H NMR spectroscopy. [c] Yield of isolated product.
[d] The available complex 6 was detected by UV analysis. [e] Based on the residual complex 6. [f] Relative rate of conversion. See the Supporting
Information [g] In toluene at 808C. [h] In CH2Cl2 at 458C. Ts=4-toluenesulfonyl.
ulating the (R/S)-SP tag using the absence or presence of
light. The recovery of complex 6 from its products is
demonstrated in Figure 3. After the RCM reaction, the
solvent was removed under vacuum. Cyclohexane and a
mixture of glycol and methanol (2:1, by volume) were later
added to the residue. At this time, the catalyst, products, and
unchanged dienes were dissolved in the upper (cyclohexane)
layer. The system was then irradiated with light to transform
the (R/S)-SP tag into the trans-ME tag, thus producing
catalyst 7, which completely shifted into the lower layer
(glycol/methanol). The products, which remained in the upper
(cyclohexane) layer, were subsequently separated. As some
polar products, such as compounds 11, 13, 15, 17, 19, and 21
(see Table S1 in the Supporting Information), were partly
dissolved in the polar phase, four to five extractions were
necessary to remove them completely. For less polar products,
such as compounds 23, 25, 27, 32, and 34 (see Table S1 in the
Supporting Information), two to three extractions were
sufficient to remove them completely. After elimination of
the products, CH2Cl2 (or cyclohexane) was added again to
start a new biphasic system. The catalyst was not directly
recovered from the polar phase owing to the strong hydrogen
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controlled process. The 1H NMR spectra showed that the
recycled catalyst complex 6 was pure (see the Supporting
Information). Therefore, 6 can be directly used for numerous
applications. The recovered catalyst exhibited good catalytic
activity, which was comparable with the parent catalyst. The
recovered catalyst retained almost the same activity for six
cycles performed under similar reaction conditions (Table 2).
Inductively coupled plasma mass spectrometry (ICP-MS)
analysis of product 11 after the first recycle using catalyst 6
(1 mol% of Ru) indicates only 9.245 ppm of Ru contamina-
tion in the crude product (see the Supporting Information),
which is similar to results obtained by previously reported
methods.[13b,22]
In conclusion, an (R/S)-SP ruthenium–carbene catalyst 6,
which can be recovered using a light-controlled process, has
been developed. This method of recycling and recovery of
homogeneous catalysts offers significant advantages because
it uses light—a cheap and clean resource—as the driving
Figure 3. Schematic for the separation of homogeneous catalyst 6
from the products.
Table 2: Recycling of 6 for RCM of dienes 10.[a]
bonding. The mixture was stored in
the dark for 3–5 minutes (7–8 h for
cyclohexane) to allow the trans-ME
tag in complex 7 to convert into the
neutral (R/S)-SP-tagged complex 6,
which then shifted back to the
bottom (CH2Cl2) layer. Removal
of solvent from the separated
bottom layer produced the recycled
catalyst, complex 6. In this manner,
the RCM products were obtained in
excellent yields (80–95%), and
good recovery of the catalyst was
achieved (94–97%; Table 1). As
some of the catalyst decomposed
Substrate Product Ru cat.
[mol%]
Cycle
TOF hÀ1[b] (convn. [%])[c]
5 min 8 min 10 min
3 min
Final
1
2
3
4
5
6
620 (34) 575 (48) 569 (76) 551 (92) 490 (98)[d]
602 (33) 575 (48) 561 (75) 545 (91) 447 (97)[e]
602 (33) 575 (48) 554 (74) 545 (91) 415 (97)[f]
565 (31) 563 (47) 531 (71) 533 (89) 415 (97)[g]
547 (30) 539 (45) 524 (45) 527 (88) 387 (97)[h]
437 (24) 527 (44) 516 (69) 515 (86) 366 (98)[i]
1.0
[a] Reaction conditions are the same as those in Table 1. [b] TOF is the average turnover frequency
measured within 3–16 minutes of the reaction starting (TOF=TON/time). [c] Conversion was
1
determined by H NMR spectroscopy. [d] At 12 min. [e] At 13 min. [f] At 14 min. [g] At 14 min. [h] At
15 min. [i] At 16 min.
during the reactions, recovery were based on the observed
residual catalyst after the reactions. The catalyst decomposi-
tion products were retained in the polar layer and were
readily separated from the catalyst.
force. It also requires no temperature changes. Moreover, it
does not require expensive supporting media. Thus, this
method can be an attractive and viable alternative for
industrial applications. With the increasing interest in the
development of recyclable catalysts for different tasks in
organic synthesis, this strategy can be applied to the design of
other transition-metal-based catalysts, template-synthesis,
and other related research areas.
Compared with previously reported solid-[18] or polymer-
supported ruthenium–carbene catalysts,[19] no supporting
media were necessary during the catalytic reaction with
complex 6. Thus, the numerous problems caused by the
supporting media that were encountered in other methods,
such as heterogeneity or slower reaction rates, were avoided.
The light-sensitive phase tag is an organic group, and it has no
effect on the catalytic activity of the ruthenium–carbene
catalyst. The catalytic reaction and the separation were
performed in a common organic solvent system. Hence, it is
more desirable than the ionic-tagged[13,20–22] or fluorous-
tagged ruthenium–carbene,[5] which have to be recycled
using ionic liquids or fluorous solvents, respectively. The use
of light, a clean and environmentally friendly resource, as the
driving force to alter the polarity of the catalyst makes this
method more viable than ferrocene redox-switchable tagged
ruthenium–carbene catalysts—these require oxidizing and
reducing reagents to switch the polarity of the catalysts.
Comparing the various methods of recovery, the current
method enabled the separation of pure homogeneous catalyst
from the products by simple extraction using a light-
Received: October 27, 2009
Revised: March 31, 2010
Published online: May 11, 2010
Keywords: carbene ligands · homogeneous catalysis ·
.
light-controlled phase tag · olefin metathesis · recycled catalyst ·
ruthenium
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